People like to see. The eye comprises several tissues that allow a person to see, and these tissues include the cornea, the lens and the retina. The cornea and lens focus light rays on to the retina so as to form an image on the retina. The cornea comprises an outer tissue of the eye that is coupled to air with a tear film, such that a majority of the focusing power of the eye is achieved based on the shape of the cornea. The retina comprises photoreceptors that generate neural signals in response to the light image formed on the retina, and these neural signals are processed and transmitted to the occipital cortex of the brain such that the person perceives the image.
The cornea is a highly innervated tissue that comprises several layers including an epithelium disposed under the tear film and a stromal layer disposed under the epithelium. In humans and at least some animals a Bowman's membrane is disposed between the epithelium and corneal stroma. The innervation of the cornea can be useful and help the person to blink so as to replenish the tear film for vision and to maintain a healthy corneal epithelium. The innervation of the cornea can also help to protect the cornea and the persons sight with the sensation of pain, such that in at least some instances the person may be forced to protect the cornea and eye from further injury in response to a painful stimulus. However, this innervation of the cornea, may result in substantial pain following surgery in at least some instances.
Many surgeries and therapies of the eye are directed to the treatment of the cornea, and in at least some instances significant pain can occur. For example photorefractive keratectomy (hereinafter “PRK”), laser assisted in situ keratomileusis (hereinafter “LASIK”), and laser assisted epithelial keratomileusis (hereinafter “LASEK”), each reshape the cornea of the eye so as to improve the focus of images on the retina such that the patient can see better. Unfortunately, many of the corneal surgeries result in pain in at least some instances. For example, with PRK and LASEK, the epithelial layer of the cornea is removed so as to expose underlying tissue that is ablated, and in at least some instances patients experience pain when the epithelium regenerates over the ablation. With LASIK, a flap of tissue comprising the epithelium and stroma is cut with a laser or blade and opened with a hinge so as to expose the underlying stromal bed where the ablation is performed. As the LASIK flap can be positioned over the ablated stromal bed with stroma to stroma contact, LASIK can result in less pain for patients. However, in at least some instances LASIK can result in complications related to the cutting of the LASIK flap and the LASIK ablation of the exposed stromal bed that extends deeper into the cornea than PRK and LASEK ablations. Also, work in relation to embodiments of the present invention suggests that the cutting of corneal nerve fibers with the LASIK flap can result in decreased corneal sensitivity for an extended time in at least some instances. Although LASIK can result in complications in at least some instances, many patients prefer the risks of LASIK to the pain of PRK.
Although the control of pain with PRK and LASEK has been proposed and implemented, many patients who undergo PRK report pain and photophobia in at least some instances during the two to four day period when the epithelium regenerates over the ablation. For example, although the use of anesthetics such as lidocaine and proparacaine have been proposed, use of these anesthetics in amounts that significantly reduce pain may delay reepithelialization, such that the safely prescribed dosage does not sufficiently reduce pain in at least some instances. Even with the use of safe amounts of analgesics with PRK and LASEK, patients can still report undesirable pain in at least some instances. Although the systemic use of opioids such as morphine can reduce pain, the patient may be subjected to side effects of the systemic opioid medication. Therefore, there is a significant unmet clinical need to reduce pain associated with removal of the corneal epithelium, for example following PRK, such that the patient is not subjected to significant side effects.
In light of the above, it would be desirable to provide improved methods and apparatus for pain control of the eye. Ideally such methods and apparatus would be compatible with refractive surgery, such that patients can receive a safe treatment to correct vision with full recovery of corneal tissue and neural function, and decreased pain.